There is a cool extension for Visual Studio, called Tabs Studio, which merges tabs whose filename and path is the same, but whose extension is different. On each tab it displays the filename without extension, followed by the extensions as clickable buttons with the tab (which is how you access each individual file). The extension which is currently being shown in that tab is highlighted in bold (although, I believe this is customizable) This helps to reduce the number of tabs open, while making it much easier to keep track of related files (eg. the .c source and .h header file)

Would be great if EmEditor could do this, although I assume a custom tab control would be needed (but it shouldn’t be too hard)!